This invention relates to the use of certain nanoscale particles as a sorbent to remove mercury from flue gas. Under the EPA's Clean Air Mercury Rule, coal fired power plants are required to drastically reduce the amount of mercury (Hg) emissions within the next several years. One of the technologies under consideration for removal of Hg is the use of chemically treated (brominated) activated carbon. It has been noted that utility companies may need to take into account the impact of a recent court decision, which specifies that a power plant cannot implement a mercury control solution that could potentially increase the amount of a secondary pollutant, unless additional controls for that pollutant are installed. This could be an issue in the case of brominated activated carbon, as bromine emissions can have adverse environmental effects. Compared to chlorine, bromine is considered to be more potent in depleting the atmospheric ozone layer. There are additional corrosion issues related to the presence of bromine in the system. Further, a majority of these activated carbons are not concrete-friendly, i.e. the fly ash containing the activated carbon particles cannot be used in concrete. This leads to loss of revenue to the power plants on two counts: (i) loss of revenue due to lack of usability of fly ash; and (ii) cost of disposing unusable fly ash in landfills. Additionally, the use of fly ash has an important consequence for the environment: if all of the fly ash produced can be used as replacement for cement, it can reduce CO2 emissions equivalent to that generated by 25% of the world's automotives.
Coal fired power plants constitute ˜52% of the total electricity produced in the United States. As the demand for electricity increases, utility companies are increasing the generating capacity as well. Additionally, many of the current nuclear plants will be “retired” in the first quarter of the 21st century. Due to poor public support for nuclear energy, these nuclear plants are likely to be replaced by coal fired plants. At the current consumption rate, it is estimated that the world has ˜1500 years of coal reserves. This leads to the recent steady increase in the amount of coal consumed in the world and in the US. This implies that the mercury emission issue associated with coal-fired power plants needs to be resolved in the long run.
An estimated total of 48 tons of mercury is emitted every year in the US from coal-fired power plants, which is ⅓rd of the total mercury emissions per year in the US. On a worldwide scale, this is a much larger issue, since countries such as China and India are using increasing amounts of energy derived from fossil fuels. Under the Clear Skies Initiative, the target is to reduce mercury emission by about 45% by 2010, and about 70% by 2018. New technologies will need to be developed to reach these targets. According to DoE, the market penetration for mercury emission reduction technologies is an estimated 320,000 megawatts. In order to achieve the target reduction by 2018, the additional annual cost for energy generation will be $2 billion to $6 billion per year, if the existing activated carbon (current estimate is $18,000-$131,000 per pound of mercury removal, using activated carbon technology.
A major issue is the usability of fly ash containing mercury adsorbed activated carbon (it cannot be used if the mercury content is high), which further increases the cost of using activated carbon technology for mercury removal. Fly ash is a valuable by-product from coal-fired power plants. In making concrete, cement is mixed with water to act as an adhesive to hold strong aggregates. Fly ash is added during the process, as it is observed that concrete containing fly ash is easier to work with, and it uses 10% less water. Additionally, fly ash reacts with lime that is given off by cement hydration, creating more bonding agent to hold the concrete together, which makes concrete stronger with time, compared to concrete without fly ash. Further, it reduces the amount of cement required to make concrete. While a ton of cement costs $80-$100, fly ash costs only $32/ton making it more competitive than cement. Manufacturing one ton of cement requires 6.5 million BTUs of energy, and it is estimated that cement plants produce 7% of the total CO2 emission by human sources. If all the fly ash produced can be used to partly replace cement in concrete, it can eliminate CO2 emissions equivalent to that of 25% of the automotives in the world. Clearly, there are environmental and societal benefits that are derived from lower mercury and CO2 emissions. Also, the use of fly ash will save landfill space. However, even the presence of less than 1% of activated carbon in fly ash can make it useless for mixing with concrete, by changing its properties.
Therefore, it is imperative that any sorbent used for removing mercury from flue gas be concrete-friendly. Conventional activated carbon is not concrete-friendly, and most brominated activated carbons are not concrete-friendly either. Recently, it has been reported that some brominated activated carbon may be concrete friendly, but the negative environmental effects of bromine are yet to be studied and not known at the moment. Additionally, bromine is a highly corrosive gas, and as such the impact on the exhaust ducts could be a problem.
Currently, various types of activated carbons are being extensively studied for mercury removal from flue gas. DOE/NETL has carried out several field tests of activated carbons due to their high removal efficiency. Three prominent brands of activated carbons which have been tested in the field are NORIT Americas (Darco® Hg-LH), Alstom Power Plant Laboratories (Mer-Clean™), and Sorbent Technologies Corporation (B-PAC™). Results indicated that activated carbon consistently performed well in mercury removal, on a full-scale test. However, secondary pollution (bromine), corrosion from bromine and concrete friendliness is still an issue, affecting their overall performance.
Another media which is used to remove mercury from flue gas is based on “clay”, and is manufactured by Amended Silicates. However, when the performance of this media was compared with various types of activated carbon sorbents the amended silicate media did not perform as well as activated carbon. Others used a fluidized bed of zeolite and activated carbon for the removal of organics and metals form gas streams. Zeolites are aluminosilicate materials that are extensively used as adsorbents for gas separation and purification, and they are also used as ion-exchange media for water treatment and purification. Zeolites have “open” crystal structures, constructed from tetrahedra (TO4, where T=Si, Al). It has been observed that the removal efficiency of metals present in gases by activated carbon is higher than that of zeolite, and the temperature only slightly influences the removal efficiency. A study tested treated Zeolite and observed 63% mercury removal efficiency.
U.S. Pat. No. 6,610,263 is directed to the use of high surface area MnOx to remove Hg. It is claimed that it has the capability to remove 99% of elemental Hg and 94% of the total mercury content in flue gas. However, the cost is likely to be a concern for using this media in practical applications.
Biswas et-al [T. M. Owens and P. Biswas, J. Air & Waste Manage. Assoc., n46, 1996, p 530] have developed a gas-phase sorbent precursor method, where a high surface area agglomerated sorbent oxide particle is produced in situ in the combustor. These sorbents are stable at elevated temperatures and provide a surface of metallic vapors (for condensation) and reaction. They used titanium isopropoxide as precursor, which decomposed at elevated temperature and formed particles of titania. Hg vapors were found to condense on these particles in the presence of UV radiation which helps in the oxidation of mercury vapors and formation of a strong bond between mercury and titania. They [P. Biswas and M. Zachariah, “In situ immobilization of lead species in combustion environments by injection of gas phase silica sorbent precursors”, Env. Sci. & Tech., v31, n9, 1997, p 2455] also used silica precursors for the removal of lead from flue gas, and were able to get 80-90% lead removal efficiency. The removal efficiency was found to be a function of the gas temperature. Additionally, the efficiency was observed to decrease with increase in temperature.
Another group has shown the feasibility of using a fluidized bed for the removal of metals, such as lead, from flue gas. They used limestone, bentonite, and alumina as sorbents, and observed that the effectiveness of the fluidized bed depends on sorbent species, sorbent particle size, amount of sorbent used, metal to sorbent ratio, metal concentration in the waste, air velocity, and temperature. Smaller particles showed better efficiency compared to larger particles (particle range 400-700 μm). In case of limestone, it increased from 60% to 70% when the particle size was decreased from 700 to 500 μm, all other conditions remaining same. The sorbents showed better efficiency at lower temperatures (˜750° C. vs. ˜900° C.). This is because at higher temperatures, the vapor pressure is high, so more metal escapes as vapor.
Still others have used zeolite materials for the removal of mercury by duct injection. They were able to get between 45 and 92% metal removal depending upon the amount of sorbent injected and the type of sorbent. In the case of zeolites, there was no effect of temperature on the removal efficiency.
Gullet et-al [B. Gullet and K. Raghunathan, “Reduction of coal based metal emissions by furnace sorbent injection”, Energy & Fuels, v8, 1994, p 1068] demonstrated the feasibility of using oxide minerals such as limestone, kaolinite, and bauxite as sorbents for toxic metal removal, by injecting them through the burner. They were able to get reduction in submicron size metal particles of antimony, arsenic, mercury, and selenium by hydrated lime and limestone.